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tying in a floating slab to a stem wall

semipro | Posted in Construction Techniques on October 22, 2008 03:05am

I’m building a shop and currently working on the foundation.  The site is pretty challenging with a slope and bedrock present.  Because of the slope, one supporting stem wall will also act as a retaining wall with about 5′ of exposure above the exterior grade.  This stem wall is built with 12″ blocks and sits on a 24″ x 8″ footer that lies below the frost line.  Rebar has been used throughout for vertical and horizontal reinforcement and I’ll likely fill all voids with grout and will have vertical rebar for reinforcement in excess of what code requires (better than every 4′).  I’m probably overbuilding the wall. 

I should add that 4 stem walls tie in to the retaining wall at 90 degrees and cross to connect to the wall that lies parallel to the retaining wall.  These walls contain horizonatal rebar that ties in to the retainin wall.  There are also 2 small deadmen walls that will be buried.  These walls should help support the lateral loads place on the retaining wall.  I’ve attached a picture called “footer” to illustrate. 

The floor of the shop will be a 4″ reinforced concrete slab that will “float” over compacted soil and gravel substrate with a sand layer and probably foam board directly under the slab (I may heat the shop with in-slab hycronics).  I do expect some settling but will compact before pouring and expect the sand and foam to give a little if settling occurs, letting the slab drop. 

I’ve attached a drawing (courtesy of Oak Ridge National Laboratories) that illustrates pretty closely the configuration I plan to build except that the 8″ block exposure above grade is more like 5′ in my situation. 

Not shown in the drawing is any kind of structural connection between the slab and stem walls and most people don’t install any.  I’ve attached a 2nd drawing that shows a possible way to connect the slab and stem wall so that the slab can still float vertically while also providing additional lateral support for the stem/retaining wall.  I believe I’ve seen this detail specified elsewhere but can’t find it now. 

What I’d like to know is what the resident experts at Breaktime think about tying the wall and slab together like this.  Additional advice on anything else I’m planning to do would be great too.

Sorry if my nomenclature is off.  I’m not a pro but want to do this right. 

Thanks in advance, Semipro

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Replies

  1. JeffinPA | Oct 22, 2008 03:19am | #1

    My sense of a floating slab that is locked into the wall is that if the wall does have pressure on it the sleeve wont allow the slab to move vertically.

    My gut is, with all the effort, just run some piers down to undisturbed soil midspan of the slab to prevent the slab from settling and build grade beams to support slab.

    Then you can pour the slab into the webs of the block after stuffing in some rebar and lock the wall to the slab. 

    I did a similar repair several years ago, but the rebar was epoxied  to a failed poured foundation wall and the garage slab was locked in to prevent the wall from moving again.

    Same concept but you would be using webs filled with concrete instead of epoxy.

    1. semipro | Oct 24, 2008 02:28am | #2

      Thanks for your response.  Your point about the rebar binding in the sleeve is well taken as are your thoughts on supporting the slab.  I'd still like to hear from others though if anyone is listening.

      Thanks.

  2. Catskinner | Oct 24, 2008 02:55am | #3

    From everything I've read and can infer from your description, it sounds like you are doing a great job.

    I also have my doubts about turning the rebar and using it as a sleeved dowel, while it might look great on paper, I think actual site conditions, no matter how careful you are, could be a different matter. My sense is you could be putting a stress riser in that slab, that is, hurting more than helping.

    From the description of your walls, I don't think they need the additional restraint of tying in the slab. Other than problem soils under the footings, I am having a hard time coming up with a scenario that would cause this structure to move.

    If the subgrade below the slab is properly compacted and leveled, there is no reason to expect settling. If the soil is good, and you've achieved 95%, then it can't go anywhere. Slabs simply do not weigh that much.

    The only detail I would consider modifying is this;

    I have had good results compacting and leveling the subgrade, putting down the XPS (extruded foam) that is rated for this use, and then putting a little bit (an inch or two) of sand or crusher fines over the foam, and then the concrete.

    If the subgrade is good, and the foam is rated for this use, it isn't going anywhere. Clean sand is nearly to density when you place it, a light shot of water will finish the job, and the additional moisture will give you a good (more even) cure on the slab.

    The sand cushions a lot of shock in the curing process -- thermal. moisture, temperature, friction, all in all it's good to have.

    Take the weight of cured concrete per cubic yard, from that figure out what a square foot of your 4" slab weighs, divide that by 144, and you'll see that in terms of PSI slabs are pretty light compared to what a properly prepared subgrade can bear.

    1. DaveRicheson | Oct 25, 2008 04:41pm | #12

       also have my doubts about turning the rebar and using it as a sleeved dowel, while it might look great on paper, I think actual site conditions, no matter how careful you are, could be a different matter. My sense is you could be putting a stress riser in that slab, that is, hurting more than helping

       

      Rebar, as he has it drawn, will do that even with his sleave. It might be possible to keep the sleave from getting any cocncrete it it, but that is an awfull iffy chance to take.

      In situations where one concrete wall or slab needs to be able to move a little in relation to another we have used smooth  bar dowels. Our most common use is slab to slab connections where we drill and epoxy one end of the dowel in place on an existing slab, then liberally grease the exposed end that projects into the new pour.

      In his case the greased section of a bent smooth dowel would be  inserted in the holes drilled in the top of the footing. This would allow for some up and down movement of the slab while giving it resistance to the lateral force placed on it from the bottom of the wall.

      I'm not sure how well this really works, as I have never seen any movement in a well compacted (modified proctor) and tested base. However, I have seen the same detail speced by a couple of different engineering firms.

      Wadya think?

      1. Catskinner | Oct 26, 2008 01:48am | #14

        I think you're right.Something else bothers me about this design, too. A floating slab is a reliable installation partly because it can move up or down relative to the stemwall. That is, the slab can remain still and the stemwall can move, or the stemwall can remain still and the slab can move, or they can both move, and they are free to do so anywhere in the structure.In this design as it is drawn, there are quite a few scenarios that would put the slab under tension, either at the top or at the bottom. That is guaranteed to end up as a break.As I'm seeing it, there is simply no good reason for this design. If everything holds still, you don't need it, if anything moves, something is going to break.If restraining the top of the wall is for any reason a consideration, then the OP would be better off with the haunched slab pinned to the stemwalls, as has been pointed out earlier.

  3. Catskinner | Oct 24, 2008 02:56am | #4

    One afterthought;

    <<Because of the slope, one supporting stem wall will also act as a retaining wall with about 5' of exposure above the exterior grade. >>

    How much of the wall is below grade?

    1. semipro | Oct 24, 2008 03:07am | #5

      Thanks for the reply Catskinner.  The bottom of the 8" thick x 24" wide footers is about 24" below grade per our local code requirements. 

      I'm starting to think like you and Jeff do that the retaining wall may be strong enough not to need further support from the slab especially since I plan to grout all voids and probably will put rebar in every core.  I was just thinking that the weight and strength of the the slab could provide additional lateral reinforcement for the retaining wall.  I really don't ever want to see that wall bow or move. 

      I'd never thought of sand above the foam board though I had planned to put it below.   I was somewhat worried about damaging the board as I place rebar and concrete above it. 

      Anyone know a good inexpensive source for foam board.  At around $30/sheet at the home centers that'a a lot of money for what basically amounts to molded coffe cups. 

      1. Catskinner | Oct 24, 2008 05:35am | #7

        The reason I asked how much is below grade is I'm trying to get a sense of how this wall is going to act under load.Your concerns are not unfounded, but I think there is a simple and inexpensive solution.If you add about a foot to the width of the footing under that tall wall so your footing is about 3' wide, and put that extra width under the building, use three longitudinal bars on high chairs and three on low chairs, tie transverse bars about every 32 inches, and make sure your verticals are tied into the footings such that the force against the wall is transferred into the rebar in the footing (someone will come along sooner or later and talk about moment and overturn), there is no way that wall is going anywhere.Then you don't need to worry about the top at all.

      2. Catskinner | Oct 24, 2008 05:38am | #8

        <<Anyone know a good inexpensive source for foam board. At around $30/sheet at the home centers that'a a lot of money for what basically amounts to molded coffe cups. >>I'd try a real supply yard instead of a home center.Home Depot makes my rabies act up, but that's just me.

  4. WillieO | Oct 24, 2008 03:29am | #6

    I personally don't like any of the details you showed. I did my shop slab poured atop the block. With a horzontal rebar tied into the vertical rear that was coming out of the block. Unfortunately I don't have any digital plans only hard copy blueprints. The slab was 5" but haunced to 8" at the perimeter.  I also used radiant hydronic heating tubes in the slab which I highly recommend. Heres a link to some photos of the work in progress. I really wish I took more photos Theres 2 pages here. Hope this helps

    http://woodworkers.us/forum/index.php?action=gallery&g2_itemId=278

     

    1. Catskinner | Oct 24, 2008 05:41am | #9

      There is nothing inherently better or worse with either approach, they just serve different functions.I've built both ways depending upon what is required. The OP started out by saying he wanted a floating slab, so taking that as a given, there are some ways to make it work.

  5. User avater
    Matt | Oct 25, 2008 03:41am | #10

    A few comments:

    1) don't place your concrete directly on dry sand.  It will suck the water out of your concrete and weaken it.  If you want to use sand, wet it thoroughly before the placing the crete.  Personally though, I prefer gravel.  Either that, or have a vapor barrier between the concrete and the sand.  You need a vapor barrier anyway...

    2) You have what amount to grade beams under the slab.  One of the basics of a floating slab is you need UNIFORM support - which you will not have.   I'd say you need to really strengthen the slab with min #5 rebar and probably an extra inch of concrete to be sure that all the bar is not too near either the top or bottom surface of the slab.  I'd put wire (WWF) in there too.   Wire mats (rather than rolled wire) lays flatter.  Plastic 'chairs' will help too.

    3) regarding foam, a few things to be aware of: a) it comes in different PSI ratings.  Since it sounds like you may have foam between your grade beam and perimeter support and the slab, it might be worth getting the better stuff.  b) rigid foam also comes with insectiside treatment if you are in a termite or other insect prone area.

    Finally - this stuff ain't cheap.  Here, spending 10k on a monolithic pour house slab is normal stuff.

    1. semipro | Oct 25, 2008 03:53pm | #11

      Great feedback Jeff, Catskinner, and Matt.  Based on what you've said, and its obvious you have experience with this sort of thing, I plan to add additional support and strength to the slab.  I like the idea of the concrete grade beams and reinforcing the slab.  I had planned to use no. 3 rebar with fiber reinforcement.  I hadn't really thought about rebar size since all previous slab work I"ve done was much less failure critical than this.  I always use chairs to positiion the rebar.  Larger rebar and a thcker slab make sense though. 

      I figure I already have a lot invested, especially in time, so spendiing a little more to decrease the risk of failure makes sense to me. The way I'm thinking I need either uniform or no settling of the slab.  What I sorely want to avoid is differential settling and an uneven slab with cracks. 

      Good point on foam compression strength.  I had thought about that and even wondered if going with the lower strength foam would allow some "give" between the slab and support beams and stem walls.  I evisioned the foam crushing in areas with support as the soil underneath settles, providing an uniform upward pressure ont the slab.   

      Thanks to all.

      1. User avater
        Matt | Oct 26, 2008 01:00am | #13

        >> I had planned to use no. 3 rebar with fiber reinforcement.  <<

        1st - I know that home centers sell #3 rebar but I think that is just because it is cheap and their target customer might not know the difference...

        2nd, IMO "fiber reinforcement" is not reinforcement at all but rather helps with surface problems like plastic shrinkage cracks or spalding.  In my area we have all but stopped using fiber - makes for less uniform finishes and doesn't seem to avert significant cracking....

        1. Catskinner | Oct 26, 2008 01:56am | #15

          <<<< >> I had planned to use no. 3 rebar with fiber reinforcement. <<1st - I know that home centers sell #3 rebar but I think that is just because it is cheap and their target customer might not know the difference.. >>>>Good catch, Matt. Semipro, take heed. The cross section of #3 is 0.110 square inch, and the cross section of #4 is 0.19 square inch.Almost twice as much steel there. The difference in strength is tremendous, the difference in price is not.If you are planning on using rebar, make sure you get the right thickness of concrete on both sides, and watch the weather and temperature when you pour.Matt is also right about finish quality. If cracking is going to be a problem for you, it's worth thinking about -- while the finish consistency is not as good, it does help quite a bit with cracking.Above all, rebar and remesh do not perform the same function. If this is not clear, ask and we'll go over this.

      2. Catskinner | Oct 26, 2008 02:01am | #16

        <<I hadn't really thought about rebar size since all previous slab work I"ve done was much less failure critical than this.>>What sort of loads do you anticipate? << I always use chairs to positiion the rebar. Larger rebar and a thcker slab make sense though. >>Expressed as a percentage of thickness, consider what a 5 inch thickness is compared to a four inch thickness.<<I figure I already have a lot invested, especially in time, so spendiing a little more to decrease the risk of failure makes sense to me. The way I'm thinking I need either uniform or no settling of the slab. What I sorely want to avoid is differential settling and an uneven slab with cracks. >>Very smart, that. It's a whole lot cheaper to do it right the first time. I wish my customers all understood that.<<Good point on foam compression strength. I had thought about that and even wondered if going with the lower strength foam would allow some "give" between the slab and support beams and stem walls.>>You don't wanna do that. Use XPS rated for the use.<<I evisioned the foam crushing in areas with support as the soil underneath settles, providing an uniform upward pressure ont the slab. >>You don't want that happening, either. Do not put foam under the grade beams or thickens. Just interrupt the placement of the foam at bearing points.

        1. semipro | Oct 26, 2008 04:57pm | #17

          Just a quick reply before I go out and continue laying block. 

          Agreed that fiber and rebar do not perform the same function that's why I've used both with great success in past applications.   Also, agreed that no 4 bar is worth the extra cost.

          Your comments on rebar are importannt and I recently found also that the no. 4 the home centers was selling was not the same grade (tensile strength) as the bar I got from a local redimix/block supplier.  I realized this when I went to cut and bend the stronger bar, ouch!  Checking the specs embedded on the bar confirmed. 

          I'm curious about concerns on putting the slab in horizontal tension though.  I would think that if properly reinforced it would be quite strong this way and that anchoring the wall with the weight of a 24'x26' slab would help stablize the wall.  The problem then becomes how to do this without introducing vertical stesses in the slab which is why I'm posting to get feedback. 

          I've included a couple of photos.  Keep in mind mind ther's a lot of bedrock that I had ot work around and that I'm uncertain about the compactness of the soil that will be under the slab.  I'm not the original owner of the property and suspect this area may have been filled.

          Thanks all.  Your input so far has helped with some positive changes to my plan.

          1. User avater
            Matt | Oct 27, 2008 01:38am | #18

            What is the hignest point of the block portion of the foundation?  4'? 5'? 6'? etc.  Are you filling the block with a pump or what? 

          2. semipro | Oct 27, 2008 08:18pm | #19

            The highest wall is 7 courses high so 56".  I plan to grout using buckets from a truck.  I know that's a pain but I'll have plenty of help.  The same group of guys has done it for one of their house foundations.

          3. WillieO | Oct 28, 2008 07:16am | #20

            Settling under the slab is always a concern. Here in the NE I use Reycled concrete aggregate (RCA) with sand on top. Its well worth it to rent a power tamper and tamp both layers. once go over the RCA thoroughly and then spred sand and tamp again thouroughly.

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